Carriers of the X-linked immunodeficiencies - severe combined immunodeficiency (SCID), Wiskott-Aldrich syndrome, hyper-IgM syndrome and X-linked lymphoproliferative syndrome-are normal by all immunologic parameters. We propose that the failure of these obligate heterozygotes to demonstrate any sign of these disorders is due to selective use of the X chromosome that does not carry the gene defect as the active X in all cell lines affected by the gene defect. If this hypothesis is correct, techniques that permit the distinction of the active and inactive X chromosomes will allow identification of the cell lines affected by the gene defects and provide a basis for carrier detection assays in these disorders. We have recently developed such a technique. Somatic cell hybrids are produced between purified T cells, B cells, NK cells or monocytes from a woman at risk and a chinese hamster cell line that is deficient in the X-linked enzyme HGPRT. Hybrids, which tend to lose human chromosomes, are grown in selective media so that only those hybrids that have retained the active human X chromosome will survive. DNA from these hybrids is analyzed, using an X-linked restriction fragment length polymorphism (RFLP) for which the woman is heterozygous. If the cell lineage used to make the hybrids is not affected by the gene defect, or if the woman studied is not a carrier of the disease in question, approximately half of the hybrids will use the maternally derived X chromosome as the active X and half will use the paternally derived one. However, if the woman studied is a carrier and the cell lineage studied is affected by the gene defect, then all the hybrids will use the X chromosome that does not carry the gene defect as the active X (non-random X chromosome inactivation). We intend to use this technique to determine the cell lineages affected by the gene defects listed above, and to provide carrier detection for these disorders. Our initial studies will focus on X-linked SCID for the following reasons: 1) determining the cell lines affected by this disorder will be particularly useful in clarifying the relationships between the various cell lines involved in the immune response; 2) there are no clinical or laboratory characteristics of X-linked SCID that distinguish it from other forms of SCID, making informed genetic counseling difficult; and 3) our ability to identify carriers of X- linked SCID will increase the number of informative individuals in affected pedigrees and thereby make it easier for us to map this disorder.